That was fine, right up to the DMCA got involved, now it's a little murkier.
If jail-breaking in any way (N.B. I don't own an iThingy and have no intention of) requires the use of anything Apple decide is proprietary and secret, you fall foul of DMCA and they can quite happily void your warranty.
Also, the warranty and/or guarantee is based on using the device in the manner it was meant to be used or in a manner you were advised it could be used (the latter to cover people in shops who say "can I use x to do y", if the shop says you can and in reality you cant, you're covered). Arguably, given the iThingy doesn't have a big button on it that says "Jailbreak here", you aren't using it as intended and voiding the warranty may be applicable.
However, all this depends on precisely why you want to use your warranty - if it's because the device caught fire whilst you were making a phone call, arguably the warranty should still apply, irrespective of what you had done to the thing (unless what you did was to set it on fire whilst making a call).
Warranties worked very well when devices were one use items.